Arch Formation Movie Text

Over 300 million years ago, our Earth’s surface was very different. The landmasses were drifting, colliding and transforming. As tectonic forces pushed and pulled what was to become North America, the ancestral Rocky Mountains rose.

To the west of these mountains, faulting and subsidence created the Paradox Basin. Over the next 15 million years, changing sea levels filled the basin 29 times. Each time the ocean receded, salt water became trapped in the basin. The trapped water evaporated, leaving a massive deposit of salt over 5,000 feet (1,524 meters) thick.

As time passed, the mountain range eroded. Vast amounts of sand, rock and debris accumulated on top of the salt layer. Under this tremendous pressure, the softer salt beds were forced westward. When the salt encountered deep faults, it was blocked and forced upwards.

Over the next 75 million years, an enormous salt wall two miles high, three miles wide and over 70 miles long was created. Eventually, the salt stopped flowing and a mile-thick layer of rock was deposited over it.

Then, some 60 to 70 million years ago, tectonic forces caused some of the deeper rock to bend, forming a dome. Long parallel cracks called “joints” formed in this bent rock.

Later, when the Colorado Plateau rose, the Colorado River and its tributaries eroded away most of this mile thick layer of stone. When the cracks became exposed at the surface, water seeped through the joints allowing some of the salt to dissolve. With the salt removed, the unsupported stone collapsed over time, creating Salt Valley.

At the edges of Salt Valley, some of the remaining fractured rock layers continued to erode, forming thin sandstone walls called “fins”. These fins were slowly worn down and sand collected between the closely spaced vertical walls. Slightly acidic rainwater, combined with carbon dioxide in the air, forming carbonic acid in this trapped sand. Over time, the acid dissolved the calcium carbonate that held the sandstone together slowly wearing away the rock until openings were formed. In other fins, an exposed layer of weaker rock lay beneath a stronger one. The weaker stone weathered first, undercutting the upper layers and creating a hole. In both cases, the weight of the overlying rock caused fracturing above the opening. Eventually, gravity pulled the loosened stone away creating the distinctive natural arch formations.

Water and time continue their relentless sculpting of this landscape, creating and transforming the natural wonders of Arches National Park.

Please go BACK to the ARCH FORMATION INTRODUCTION page to complete this activity.

Last updated: February 24, 2015

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